The switching is achieved by means of metal contacts, which are subjected to movements, wear and tear. The switching thus achieved is associated with use arcing and contact bouncing.
The above problems are to a major extent overcome by incorporating the solid-state devices for switching. These gave birth to the solid-state relay (SSR). Solid-state relay is defined as a relay with isolated input and output; we functions are achieved by means electronic components without the use of moving parts - NARM (USA)
It is still very common practice that switching is done by electrical contactors. The older-generation technicians are till not fully prepared to switch over to solid-state switching: either the awareness is lacking or the industry has not put in any significant efforts promote the new switches.
Capacitor switching, unlike other loads (resistive and inductive) is very severe load with respect to its inherent property of energy storage. The charge and discharge cycles are critical to handle while switching the capacitor banks using SSRs.
The three phase capacitor banks are widely used for improving the power factor and the application areas are in the power distribution business, either the conventional way or by wind energy. There are still some electrical problems in the application, which have to be tackled.
Whoever has initiated to switchover to solid-state switching show signs of satisfaction. Further efforts are being put in to solve the minor technical snags by building appropriate logic circuits. It is recommended that the users to take care of the following technical parameters while designing the system:
1. To select an appropriate SSR, kVAR rating of the capacitor should be known and the supply voltage. There is another critical parameter of capacitor called effective series resistance (ESR) which can be measured. This parameter helps to know the surge current and thereby to decide the SSR rating.
2. Depending on the SSR current appropriate extension heatsink should be used with adequate cooling fan in the control panel. It is recommended to keep the inside ambient at @ 45-50 degree C.
3. Lightening arrester is a must.
4. Inductor coils in series with the SSR is a must.